Ludlum Measurements 3100, Руководство пользователя

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4-6
I = 9.9 x 10
-13
coulombs/sec
I = 0.99 pA ( per µCi of tritium)
4.4.2 Electrometer.
The electrometer consists of summed electrodes providing a net signal to the input of a
guarded trans-impedance amplifier with a feedback resistor that provides a short time constant. A short time
constant means that the chamber reacts quickly. Large spikes in the output data are caused by radon or other alpha
emitters, which have approximately 1000 times the energy of the tritium beta particle (5.5 MeV vs 5.65 keV). These
spikes are filtered out in firmware, without affecting the tritium signal, using a proprietary algorithm.
Given a chamber volume is 250 cm
3
, and that the current is measured with a feedback resistor of 500 Gohm
(=500x10
9
ohms), then the resulting voltage output of the electrometer V, per microcurie/m
3
is:
V = (0.99 x 10
-12
) (250 x 10
-6
)(500 x 10
9
) = 0.124 mV per µCi/m
3
The voltage output of the electrometer board is then connected to the main board for further processing.
4.4.3 Main Board Radiation Signal Processing.
4.4.3.1 Amplifier Range Change. The microprocessor on the main board controls the amplification of the
electrometer signal. The Model 3100 is auto-ranging so the user doesn’t have to switch a range control as the
radiation level changes, but internally, the microprocessor controls three ranges to properly adjust the amplifier gain.
The microprocessor starts in the X1 range, which covers from 1 to 100 µCi/m
3
(0.037 to 3.7 MBq/m
3
). When the
chamber reaches 100 µCi/m
3
(3.7 MBq/m
3
), the output of the electrometer is 12.4 mV relative to guard potential,
and the gain on the main board is switched to the X10 range. When the chamber reaches 1000 µCi/m
3
(37 MBq/m
3
),
the output of the electrometer reaches 124 mV relative to guard potential, and the gain on the main board is switched
to the X100 range. The top calibrated reading is 10,000 µCi/m
3
(370 MBq/m
3
) when the output of the electrometer
reaches 1240 mV relative to guard potential.
4.4.3.2 Offset Adjustment.
The electrometer operational amplifier (op amp) will have some offset voltage. An
internal calibration control on the main board marked OFF provides an offset voltage that can be adjusted during
calibration to null out the electrometer offset voltage.
4.4.3.3 Analog-To-Digital Input. The voltage from the amplifier section of the main board then enters the
analog-to-digital (A/D) input of the microprocessor. The microprocessor samples and measures this voltage input
every 200 milliseconds. It compares this input to the 2.5 V reference voltage and digitizes the result. The
microprocessor uses proprietary digital filters to average the results and then converts the readings into the
calibrated displayed reading.